Abstract : The objective of this study was to identify helicopter characteristics critical to main rotor flapping and to attempt to establish flapping design criteria. Three helicopter types and three rotor systems were simulated in steady flight an maneuvers using the hybrid version of C81, Rotor craft flight Simulation Program. Critical operational characteristics were: at center of gravity extremes, under low or negative g conditions, with large abrupt control inputs, and in conditions of significant retreating blade stall. Operation outside recommended flight envelopes can cause excessive flapping. Helicopter characteristics influencing flapping were: flapping restraint, fuselage stability characteristics, and helicopter loading conditions. A limit flapping criterion is defined the same as current design specifications and should apply for all operations within the recommended flight envelopes of the helicopter. An ultimate flapping criterion is proposed for operations outside the recommended flight envelopes and for failure conditions. The ultimate flapping criterion requires no failure of primary structure due to flapping stop contact, and no rotor blade contact with the fuselage for conditions where probability of occurrence is not extremely remote. (Author)